Micro-porous superlattice separations

ABSTRACT

A high surface area substrate with controlled pore size and slot geometry is used in an adsorbing process. The material is made by depositing at least two materials in alternating layers. The film is then broken up and one of the materials is etched away to produce a slotted surface structure. These slots can add size and shape selectivity to separations and catalytic processes which because of the uniform and controllable dimensions (&gt;5A) would be superior to that obtainable from zeolites and clays.

BACKGROUND OF THE INVENTION

Materials with controlled size pore structure on the atomic dimensionhave been used as molecular sieves for sorption, catalysts, and ionexchange resins. The most well known of these materials are zeoliteswhich is a name derived from the Greek, meaning boiling stones. Thecontrolled pore structure of zeolites result from the chemicalarrangement of (Al,Si)O₄ tetrahedra which share all their oxygenvertices with nearby tetrahedra and are joined together to give rise tolarge cavities and controlled size windows into these cavities. Thealumino-silicate framework forming the zeolite usually has a negativecharge which is balanced by alkalies and alkaline earths located outsidethe tetrahedra in the channels. These materials have proven to be usefulin a variety of industrial applications because of the shape andchemistry of the pore structures formed by the linked alumino silicatetetrahedra. Pores in these materials can be monodisperse and smallenough to act as molecular sieves so that different apparent surfaceareas are obtained according to the size of the absorbate molecules. Thechemistry of the framework and the counter ions neutralizing electricalcharge on the framework can have many catalytic applications. Inparticular, the controlled pore structure can produce shape selectiveeffects in catalysis (see, e.g., N. Y. Chen, U.S. Pat. No. 3,630,966,Dec. 28, 1971).

In all zeolites, the chemical composition is intimately related to thesize of the pore structure. Maximum pore size in zeolites is related tothe geometric arrangement of the alumino-silicate tetrahedra and isalways less than about 10 Å.

The present invention describes zeolite like materials for use asmolecular sieves. These materials are made using physical fabricationtechniques. Physical fabrication techniques such as etching, depositionand lithographic patterning have been extensively used for theproduction of microelectronics. Features with approximately one microncritical dimensions are routinely created using these methods formicroelectronic circuitry; however, to produce porous materials capableof molecular sieving requires reducing the feature size by three ordersof magnitude.

Reduction in feature size is obtained by using new methods for definingthe pattern used with thin film etching and deposition techniques. Byusing physical fabrication techniques to produce controlled porosity onthe molecular dimension, new degrees of freedom in constructing zeolitelike materials are obtained. Physical fabrication techniques decouplethe interrelationship between size and chemistry of the pore structures.Thus, the composition of zeolite like materials made with physicalfabrication techniques is not limited to aluminosilicates. Using thephysical fabrication techniques described herein it is possible to makezeolite-like materials from a wide variety of semiconductors, metals andinsulators. Shape of pore structures made with physical fabricationtechniques can be significantly different from those in natural andsynthetic aluminosilicate zeolites. For example, the pore structureoccurring in the physically fabricated etched superlattice structuredescribed herewith is two dimensional rather than one dimensional as isthe case for conventional zeolite materials. Precise control of poresize can be obtained using physical fabrication allowing for a choice ofcritical dimensions in the size range from approximately 10 to more than10,000 Å. This is a size range not readily accessible with conventionalzeolite materials. With these broad ranges of flexibility ofconstruction, etched superlattice zeolite-like materials made byphysical fabrication techniques have a control over pore size andchemistry which is not available with conventional aluminosilicatezeolites. Since pore size and chemistry are the determining factors inuse of zeolites for separations and catalysis, etched superlatticezeolite-like materials will have many inherent advantages in theseareas. In particular, etched superlattices provide a new class ofmicro-porous shape selective materials that significantly expand therange of behavior spanned by conventional zeolites. The term "shapeselective" or "shape selective activity" is taken throughout this patentto mean a material whose interaction with chemical molecules can bedifferent depending on the molecular size or shape.

SUMMARY OF THE INVENTION

The present invention is a process for absorbing a molecular speciesfrom a medium comprising contacting the medium with a superlattice whichincludes a multilayered material having layers which extend laterallywith different dimensions so as to provide two dimensional slots withinthe multilayered material.

Absorption includes the penetration of the molecular species into theinner structure of the superlattice as well as adherence of the speciesto the surface of the superlattice.

In preferred embodiments, the medium may be a liquid or a gas. Themolecular species may be organic, a hydrocarbon or aromatic. Specificexamples include tetracene, pyrene, anthracene, naphthalene, rubene anddipyrenyl decane.

The absorption of the species onto or into the superlattice separatesthe species from the medium. Removing the superlattice from the mediumthen removes the species from the medium. The absorption of the speciesby the superlattice may depend on the size or shape of species. Forexample, a smaller molecule may be separated from a larger molecule bythe smaller molecule preferentially entering the slots of thesuperlattice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a fabrication sequence used tocreate controlled dimension pores in superlattice zeolite-likematerials.

FIG. 1a shows a schematic representation of a superlattice composed oftwo alternating layers deposited on a substrate.

FIG. 1b shows a schematic representation of a superlattice of FIG. 1aafter it has been divided into cylindrical post structures.

FIG. 1c shows a schematic representation of a microporous superlatticematerial formed by selectively etching the exposed layers shown in FIG.1b.

FIG. 2 illustrates the use of an interfacial layer for removing apre-divided superlattice film from a substrate.

FIG. 2a shows a schematic representation of a superlattice deposited ona substrate coated with interfacial release layers.

FIG. 2b shows a schematic diagram of the superlattice shown in FIG. 2aafter it has divided in the form of cylindrical post structures.

FIG. 2c shows a schematic representation of a free standing microporoussuperlattice material formed by selectively etching exposed superlatticeshown in FIG. 2b and dissolving the interfacial release layer.

FIG. 3 shows a transmission electron micrograph of the slot shaped poresformed at the perimeter of a 3000 Å diameter post that was etched intoan amorphous silicon amorphous germanium superlattice structure. Poresshown in the figure are approximately 30 Å in width.

FIG. 4 shows a transmission electron micrograph of layers appearing inpost structures formed on a sample grown with a 75 Å repeat distance.The layers are essentially flat and uniform, with the darker layer beingamorphous silicon and the lighter layer amorphous silicon dioxide.

FIG. 5 shows a transmission electron micrograph of slot structures thatwere etched into posts shown in FIG. 4. The dark central core of thepost is the unetched portion of the divided superlattice.

FIG. 6 shows a transmission electron micrograph of a micro-poroussuperlattice material encased within an 30 Å thick fluorocarbon bag.

FIG. 7 shows the fluorescence spectra of tetracene molecules trapped inslots, in solution, and in a solid film.

FIG. 8 shows the fluorescence spectra of dipyrenyl decane moleculestrapped in slots, and in solution.

FIG. 9 shows a micrograph of a slot structure before (a) and after (b)the absorption of molecules.

DESCRIPTION OF THE PREFERRED EMBODIMENT A. Preparation of SuperlatticeAbsorbers

Superlattices consisting of thin film layers 5-2500 Å thick, provide aunique template for forming two dimensional pores with preciselycontrolled surface chemistry. By breaking the thin film up in a mannerthat exposes edges of the thin film layers it is possible to create aslotted structure by selectively etching away one or more of thematerials comprising the superlattices. FIG. 1 shows a schematic diagramof a fabrication sequence used to create controlled dimension pores insuperlattice zeolite-like materials. In the sequence shown in FIG. 1alternating thin film layers 1,3 are sequentially deposited onto asubstrate 5. The lithographic template formed by the alternating layers1,3 is exposed by patterning cylindrical post structures, 7 into thefilm surface. The layers exposed at the edge of the post 9,11 form thepattern used to delineate slots in a selective etching process. When oneof the layers 9 exposed at the post edge is selectively etched, slots 13are formed in the post and the material containing the array of etchedslots is referred to as a micro-porous superlattice material 15. Thewidth and uniformity of the resulting slot is determined by thethickness and uniformity of the deposited film. Since superlattices canbe grown with layers that are flat and smooth to better than 5 Å, (P. N.Petroff, A. C. Gossard, W. Weigmann and A. Savage, J. Cryst. Growth 44,5 [1978]; H. W. Deckman, J. H. Dunsmuir and B. Abeles, Appl. Phys. Lett.46, 171 [1985]) the resulting slot structure can be smooth on atomicdimensions. Width of the slot can be as small as 5 Å, which iscomparable to the size of many small organic molecules. Larger size twodimensional pores can be produced from thicker layers to accommodatelarger molecular species. Chemistry of the slots can be directlycontrolled by the choice of materials used to form the superlattice.

Superlattice materials have interesting physical and electricalproperties (including quantum wells) which are not relevant for thepresent invention. As used herein, superlattice materials are layeredmaterials in which the interfaces between layers are sufficiently abruptand smooth so that if the layer thicknesses are made small enough,quantum wells would be observed.

For superlattices composed of two alternating layers, the walls and endsof the slot will usually have different chemical behavior. This offersunique possibilities for designing synthetic etched superlatticezeolites in which shape selective absorption is decoupled from chemicalreactions which can be catalyzed by the material forming the slot end.Moreover superlattice structures composed of three sequentiallyalternating layers provide the opportunity for producing differentchemistry on the two faces as well as ends of the slots. This approachto the formation of shape selective materials offers unparalleledfreedom in choosing size and surface functionality of the porousmaterial to optimize the performance as a molecular seive.

Specific surface area for sorption (M² /gm) in etched superlattices canbe accurately adjusted during fabrication. Depending on thecharacteristic thickness t₁ and t₂ . . . t_(n) of individual layers andthe fraction (f) of the characteristic length (D) of each layer that isetched, specific surface areas can be adjusted to be in the range of0.001-2,500 M² /gm. For superlattices divided and etched in the mannershown in FIG. 1, the specific surface area for sorption, S, is ##EQU1##where ρ₁ and ρ₂ are the density of the unetched and etched layers,respectively, and t₁ and t₂ are the corresponding layer thicknesses. Inthis case the layer length is equal to the post diameter D and the depthl of the slot etched into the edge of the post is l=fD/2 where f is thefractional etched length of superlattice in each post. When f is lessthan 1/5, Eq. 1 can be approximated as ##EQU2## and it is seen that thespecific surface area varies linearly with the fractional length ofsuperlattice etched. To obtain materials with large specific surfaceareas (in excess of 0.1M² /gm), it is desirable to have f>1/40. Anapproximately equivalent statement of this condition which is alsopreferred is to have the maximum lateral extent of the etched layers(D-2l) be less than forty times the length of the slots, l. In a morepreferred embodiment, which produces high specific surface areas(10-2500M² /gm) f is greater than 1/4. This condition can be restated inthe following more preferred form, which is that the maximum extent ofthe etched layers (D-2l) is less than 4 times the slot length, l. Whenthe superlattice is divided into irregularly shaped particles, D istaken to be an average length of each layer and l is taken as theaverage effective slot depth; and the fractional slot depth f is definedas f=2l/D. When more than two layers comprise the repeat unit of thesuperlattice, then the preferred conditions are taken to apply to theetched layers.

Superlattices of semiconductors, metals and insulators have beenproduced using a wide variety of deposition techniques. These materialswere originally produced to study optical, electronic and magneticproperties of stacks of ultra-thin layers. Initially, superlattices weremade from crystalline semiconductors by molecular beam epitaxy (seem forexample, L. Esaki and R. Tsu, I.B.M. J. Res. Dev. 14, C1 (1970), A. C.Gossard, Thin Solid Films); a procedure which made very perfect films,but which was suited only for the production of small areas and limitedvolumes. with the expansion of interest in superlattice materialsseveral other deposition techniques have been used to grow smoothsequential layers with abrupt interfaces. Techniques which have beenused to produce superlattices include chemical vapor deposition,sputtering and evaporation. Most recently, it has been shown that plasmaenhanced chemical vapor deposition can be used to produce superlatticesof tetrahedrally bonded amorphous materials (B. Abeles and T. Tiedje,Phys. Rev. Lett. 51, 2003 (1983)) such as a--Si:H, a--Ge:H, a--Si_(1-x)C_(x) :H, a--SiO_(x) :H or a--SiN_(x) :H. A key requirement for all ofthese deposition techniques is that the superlattice be deposited at atemperature below which interdiffusion of the layers occurs. Anotherequirement is that the surface mobility of the depositing species besuch that the layers do not grow as islands but rather grow as flatsmooth continuous films.

Alternating layers ae formed in evaporation and sputter depositionprocesses by sequentially translating a substrate in front of two ormore deposition sources. The thickness of each layer is determined bythe time each substrate is held in front of a deposition source as wellas by the rate of deposition. In plasma assisted chemical vapordeposition alternating layers are formed by sequentially changing thereactive gases flowed through the plasma reactor. In this case,deposition time and rate directly determine the thickness of each layer.Layer composition is determined by choice of reactive gases for plasmaassisted chemical vapor deposition and source composition for sputteringand evaporation. The independent control of layer spacing andcomposition offered by these techniques provides a degree of freedom indesigning a molecular sieve which is not available with zeolites orclays. This degree of freedom also allows for making multilayeredmaterials with predetermined thickness in each layer as well as withcompositional variations in different layers which do not necessarilyform a simple repeat structure.

For purposes of illustration of the present invention, the followingdiscussion shall describe one embodiment wherein the superlatticematerial includes layered material having a repeat distance wherein eachrepeat distance includes two or more layers. The composition of eachrepeat is then repeated throughout the superlattice. Since the methodsfor fabricating superlattices allow for adjustments of composition andthickness of each individual layer, the word repeat is taken to mean acharacteristic unit of substantially similar thickness and/orcomposition. It is noted that sequential repeat units can haveintentionally introduced differences which do not substantially affectthe performance of etched superlattice zeolite-like materials inparticular process applications.

To etch slots into the superlattice material, deposited films must bedivided in a manner that exposes the edges of the thin film layerscomprising the superlattice. A number of different methods are availablefor breaking the film up without delaminating the layers. Methods ofdividing the film to expose the layer edges include grinding,pulverizing, and lithographic patterning. Of these, lithographicpatterning techniques can produce the most uniform particle sizes. Forlithographic patterning, a mask is deposited on the surface of thesuperlattice with a pattern which defines the shapes of the particles tobe created. The pattern is transferred from the mask into the underlyingsuperlattice by differentially etching the superlattice with respect tothe mask. This process divides the superlattice film into individualparticles whose lateral dimensions are determined by the geometry of themask. For example, the film can be divided into cylindrical posts byusing a sub-monolayer of spherical colloidal particles as an etch mask(Natural Lithographic Fabrication of Microstructures over Large Areas,H. W. Deckman and J. Dunsmuir, U.S. Pat. No. 4,406,695). Diameter of theresulting post is determined by the colloidal particle diameter and thedirectionality of the etch used to transfer the pattern on the surfaceof the film. If the pattern transfer is from an anisotropic etch, thediameter of the resulting post will closely correspond to that of theparticulate etch mask. Anisotropic etches which can be used tofaithfully transfer the pattern of the spherical mask include ion beametching and directional reactive ion etching with the plasma. Althoughlithographic patterning can produce the most uniform particle size, itis in some cases desirable to divide the film using physical fracturemethods such as grinding and pulverizing. In this case the particle sizeis determined by the detailed abrasion method used, although typicallyparticles will have dimensions greater than 1 μm.

In general, it is preferable to minimize the particle size in thedivided film. By minimizing particle size, smaller fractional etchlengths, f, can be used to make high surface area materials. The surfacearea in the etched superlattice structure varies linearly with thenumber of layers exposed in the divided film structure. To produce highsurface area materials, it is preferred that the multilayeredsuperlattice include at least 5 layers and it is more preferred for itto include at least 25 layers. Furthermore, it is preferred that atleast this number of layers be exposed at the edge of the divided filmstructure.

In some cases it is desirable to remove the film from the substrate whenit is divided to expose layer edges. The substrate can be removed bydissolving it in an etchant which does not attack the superlattice. Forinstance, superlattices composed of silicon and silicon nitride can beremoved from glass or quartz substrates by etching in a hydrofluoricacid bath. Dissolution of a superlattice film from a substrate can oftenbe aided by use of an interfacial layer. FIG. 2 illustrates the use ofan interfacial layer 24 for removing a pre-divided superlattice filmfrom a substrate. The substrate 25 is first coated with a thininterfacial layer 24 before the layers of the superlattice 21 and 23 aredeposited. It is possible to remove the superlattice film from thesubstrate by simply dissolving the interfacial layer before or afterdividing. In FIG. 2 the film is removed after dividing. The superlatticefilm shown in FIG. 2 is lithographically divided into cylindricallysymmetric post structures, 27, exposing edges of the layers 29 and 31.The interfacial layer 24 can then be dissolved either before or afterslots 23 are etched into the particles 27. After dissolution of theinterfacial layer 24, the etched particles of the superlattice that arelifted off the substrate 25 form an etched micro-porous superlatticematerial, 31. For instance, a silicon/silicon dioxide superlattice canbe removed from a glass substrate by etching away an interfacial coatingof aluminum. The thickness of the interfacial layer should be such thatthe etchant can readily diffuse to lift the superlattice film away fromthe substrate.

Slot structures are formed by selectively etching the exposed edges, 9,11, 29 and 31 of the superlattice film. The pattern formed by thealternating layers at the edge of the superlattice film can be thoughtof as a mask for the creation of a slot structure in an etching process.The pattern in this mask is synthetically generated during thedeposition process. The slot sizes determined by the individual layerthicknesses and the chemistry of the slot wall is determined by thematerials used to form the superlattice film. FIGS. 1 and 2 showschematic diagrams of superlattice films patterned with an array ofmicrocolumnar posts that expose the layer edges 9, 11, 29 and 31. Slotstructures 13 and 23 are formed by differentially etching the layers inthis pattern occurring at the post edges. To obtain the type ofcontrolled slot structures 13 and 39 shown in FIGS. 1 and 2, it isnecessary for the layer being etched to etch more than 10 times fasterthan the adjacent layer. A more preferred range of chemical selectivityfor the etching process is a ratio of 100:1 for the etching rates ofadjacent layers.

From the foregoing description, it will be apparent that differentiallyetched superlattices form a new class of shape selective materials.Variations and modifications of this composition of matter willundoubtedly suggest themselves to those skilled in the art.

It is also clear that several applications of these materials exist inthe areas of separations and catalysis. By making the slot widthcomparable to molecular dimensions of interest; shape and size selectiveeffects can be introduced into separative as well as catalyticprocesses. Slot width can be made comparable with molecular dimensionseither by directly fabricating the desired width or by reducing theeffective slot width using a coating formed after the slot isfabricated. Coatings applied to the slot wall which reduce the effectivewidth can either be organic or inorganic. Suitable organic molecules forlining the interior of a slot and reducing the effective width includealcohols, surfactants, silane coupling agents . . . . The organic layernot only reduces the effective width of the two dimensional slot, butalso can be used to alter the surface chemistry. For instance, the wallof a slot can be made either hydrophobic or hydrophyllic and exhibitabsorption phenomena which are not only size dependent but are alsoselective to the hydrophobicity of a molecule. If the layer forming theslot end acts as a catalyst, then only reactants with the properhydrophobic character will be brought to the reaction site, yielding acatalytic process which is both selective with respect to size andfunctionality of the reactant. Other variations involve applying acoating layer selectively at the slot end to either create a catalyticsite or surface functionalization. Catalytic processes occurring at theend of a slot will be geometrically constrained and may tend to suppressundesired reaction channels. For instance, branching reactions occurringduring polymerization will be constrained by the slot walls. Themicroporous superlattices also give new degrees of freedom in chemicalprocess design. Since the active material can be fabricated directly ona substrate several different types of "fixed" bed processes can beenvisioned. By making the substrate out of a flexible material such aspolyimide, it is possible to make a continuous belt having microporoussuperlattice material on the surface. This belt can be moved between twotanks of liquids; transporting material trapped in the slots from onetank to another. By absorbing material in one tank and removing it fromthe slots in another, a continuous separation is produced.

Another separation method can be designed to take advantage of theplanar geometry offered by micro-porous superlattices fabricated onsubstrates. In this case a chromatographic separation column ismicrofabricated using the micro-porous superlattice as the packing. Bymicrofabricating a flow channel over a micro-porous superlattice it ispossible to obtain a micro-bore chromatography column inexpensively madeusing lithographic fabrication techniques.

The invention will be better understood by reference to the followingillustrative examples.

EXAMPLE 1

To demonstrate the fabrication of the absorbing materials of the presentinventon, a micro-porous superlattice material was prepared from asemiconductor superlattice consisting of layers of hydrogenatedamorphous silicon alternating with layers of hydrogenated amorphousgermanium. Alternating layers of hydrogenated amorphous silicon andhydrogenated amorphous germanium were deposited on polished crystallinesilicon wafers using an rf plasma assisted chemical vapor depositionprocess in which the reactive gases pure SiH₄ and GeH₄ diluted with H₂(1:10) were changed periodically in the plasma reactor withoutinterrupting the plasma. In order to achieve sharp interfaces betweenthe layers, the residence time T_(r), of gas molecules in the reactorwas made to be short compared with the time T_(m) required to grow amonolayer. The residence time for the gas is T_(r) =Vp/F₀ p₀ where V isthe volume of the reactor, p is the gas pressure in the reactor and p₀and F₀ the pressure and flow rate at STP. The condition T_(r) <T_(m) isreadily achievable. For instance, in our experiments we use V=3 liters;p=30 m torr; F₀ =85 cc/min. so that the expected value of T_(r) is onesecond. Depositions are performed using an rf power of 5 watts at 13.56megahertz and T_(m) =3-5 sec. These depositions conditions insure abruptinterfaces between layers in the growing film. Uniformity of the layersand smoothness of the interfaces is determined in part by the depositiontemperature; and in the present reactor substrates are placed on theanode and both cathode and anode (6 inch diameter separated by a 2 inchgap) are heated to 180°-220° C. Using this deposition technique filmswith more than 400 alternating layers of a--Si:H and a--Ge:H weredeposited with individual layers having thicknesses of approximately 25Å. Films were divided in the form of cylindrical posts using amicrofabrication technique called natural lithography. (H. W. Deckmanand J. H. Dunsmuir, Appl. Phys. Lett. 41, 377 [1982]). In this techniquean etch mask is formed from a submonolayer of regularly shaped colloidalparticles coated onto the film surface. Colloidal particles are coatedas a densely packed random array by inducing an electrostatic attractionbetween the colloid and film. The colloidal particles used as an etchmask are monodisperse polystyrene spheres (such as can be purchased fromDow Diagnostics) which have a negative surface charge of 1-5 μC/cm² dueto a persulfate initiator utilized in an emulsion polymerizationprocess. The a--Si:H/a--Ge:H superlattice acquires a charge similar tothat of the sol and a thin interlayer must be deposited to reverse thesign of the surface charge on the superlattice. To create a negativecharge on the superlattice, a monolayer of 100 Å colloidal aluminaparticles is deposited. The colloidal coating is produced by dipping thesuperlattice film into a pH 5 colloidal alumina sol (0.1% solids) andrinsing off those particles which are not electrostatically bonded tothe surface. After excess coating material has been removed, thesubstrate is dried with the coating fixed on the surface. This coatingimparts a positive surface charge which can be used to attract amonolayer of the monodisperse polystyrene latex. The monolayer ofpolystyrene particles is coated onto the surface in a similar manner byimmersing the film into a pH 5 polystyrene latex (0.3% solids) andrinsing off those particles which are not electrostatically bonded ontothe surface. After the excess coating material has been removed and thesubstrate dried, the monolayer of polystyrene particles is used as alithographic mask in an etching process. Cylindrical posts are createdby directionally etching the colloidally coated superlattice with a CCl₄ion beam generated in a Kauffman type ion source and accelerated to 500electron volts with a current density of 0.5 milliampers per cm². Theion beam erodes the superlattice approximately 5 times faster than thepolystyrene sphere mask creating microscopic cylindrical posts. Heightof the post structures (and thus the number of layers exposed) isdetermined by the etching time. In the present example the CCl₄ ion beameroded the superlattice for a period of ten minutes. Layers exposed atthe edge of the cylindrical post structures were differentially etchedusing hydrogen peroxide. Hydrogen peroxide selectively attacks thegermanium layer in the amorphous silicon/amorphous germaniumsuperlattice. To form slots, a densely packed random array of 3000 Ådiameter posts was etched for 15 seconds in hydrogen peroxide. Thehydrogen peroxide was removed from the etched slots by rinsing indeionized water for 30 seconds followed by a a final rise in isopropylalcohol. The isopropyl alcohol was then evaporated from the slotsleaving free standing slot shaped pores.

FIG. 3 shows a transmission electron micrograph of the slot shaped poresformed at the perimeter of a 3000 Å diameter post that was etched intoan amorphous silicon amorphous germanium superlattice structure. To viewthe slots at the perimeter of the posts, a 200 micron wide slice was cutfrom the substrate on which the superlattice was deposited and alignedin the transmission electron microscope so that the electron beam grazesacross the etched surface. The dark bands in the transmission electronmicrograph correspond to the amorphous germanium layers while the lightbands correspond to amorphous silicon layers. Gaps apparent in theamorphous germanium layers at the edge of the post correspond to theslots etched into the superlattice structure. It is seen that theaverage width of the slots is determined by the layer spacing in thesuperlattice, even though the layers defining the slots have curled.Curling of the layers at the edge of an etched slot can be due to eithercapillary effects occurring during drying of solvent from the etchedslot; or high intrinsic stresses in the thin superlattice layers exposedduring the etching process. The present invention is meant to includeslot structures which have curled as well as slot structure which remainflat. In general, the curling distorts the slot width by less than 30%on average.

EXAMPLE 2

Flat uniformly spaced slotted pores were prepared from an amorphoussuperlattice consisting of hydrogenated amorphous silicon layersalternating with layers of amorphous silicon dioxide. These layers weredeposited on polished crystalline silicon wafers using a depositiontechnique similar to that described in Example 1. The hydrogenatedamorphous silicon layers were grown by flowing SiH₄ (pure) through thereactor and the hydrogenated amorphous silicon dioxide layers were grownby flowing N₂ O₂ diluted with SiH₄ (50:1). In this case, abruptinterfaces between layers were obtained by interrupting the plasma inthe reactor for 10 seconds while the gasses were changed. Depositedfilms were divided in the form of cylindrical posts using a methodsimilar to that described in Example 1. A lithographic mask of 2,000 Ådiameter monodisperse polystyrene spheres was deposited on the filmsurface and posts were formed by reactive ion milling for approximately10 minutes with a 500 ev; 0.3 mA/cm² beam formed from CF₄ gas. Etchingwith this type ion beam can leave a 50 Å thick fluorocarbon residue onthe post which if desired can be removed by etching the posts forapproximately 2 minutes with a 0.3 mA/cm² O₂ ion beam. This residueremoval technique can also remove the remnants of the sphere mask usedto define post structures. If the residue is not removed with an O₂ ionbeam, the sphere mask remnants are removed by light abrasion appliedduring a surfactant wash.

FIG. 4 shows a transmission electron micrograph of layers appearing inpost structures formed on a sample grown with a 75 Å repeat distance. Itis seen that the layers are essentially flat and uniform, with thedarker layer being amorphous silicon and the lighter layer amorphoussilicon dioxide. The lithographic template formed by the exposed layersat the post edge is clearly visible in FIG. 4, since the fluorocarbonresidue from the CF₄ etching was removed with an O₂ ion beam. Slots werecreated from this lithographic template by etching the exposed SiO₂layers with buffered hydrofluoric acid (Timetch manufactured by TransineInc.) for 3.5 minutes. The etch was terminated by immersing the samplein dilute NH₃ OH, to neutralize the acid. Following the neutralizationstep, the sample was rinsed in water then isopropyl alcohol and dried ina flowing N₂ gas stream. FIG. 5 shows a transmission electron micrographof slot structures that were etched into posts shown in FIG. 4. The darkcentral core of the post shown in FIG. 5 is the unetched portion of thedivided superlattice. Dark bands across the post correspond to theunetched amorphous silicon layers while light bands at the exterior ofthe dark central core correspond to etched slot interiors. The slotsshown in the micrograph extend 500 Å in from the post structures and areflat and uniform.

When the residue from CF₄ etching is not removed prior to etching, thetype of slotted posts shown in FIG. 6 are obtained. In this case theetched slotted post structure is encased in a 30 Å thick fluorocarbonbag. The slotted post structures are readily visible through the thinpolymeric bag.

EXAMPLE 3

The method of Example 2 was used to divide a deposited amorphoushydrogenated silicon/amorphous hydrogenated silicon dioxide superlatticeinto 2,000 Å diameter posts. The amorphous silicon layer (rather thanthe amorphous silicon dioxide layer) was etched away using XeF₂ as anetchant. XeF₂ was delivered to the surface of the divided superlatticeas a gas at a pressure of 30 millitorr. Because XeF₂ is a highlyselective silicon etchant, the divided superlattice was cleaned with a500 ev (0.25 ma/cm²) argon ion beam for one minute prior to etching. Theetching process was conducted in the vacuum system used to generate theion beam, so that samples were not exposed to air, between the cleaningand etching steps. the XeF₂ etching process was conducted for a periodof 10 minutes resulting in a micro-porous superlattice material forwhich slots are generated by removal of an amorphous silicon layer.

B. Absorption of Molecules by Superlattice Material

Micropores were prepared from a--Si:H/a--SiO_(x) :H superlattices usingthe method as described above. In this method, a microporous material iscreated by breaking the superlattice film up in a manner which exposesedges of the thin film layers followed by a selective etching of one ormore of the thin film layers. In the present invention, both XeF₂ and HFwere used as selective etches. The deposited superlattice was divided inthe form of individual posts using a lithographic patterning process.The "posts" were not separated from the silicon substrate. The slotsetched into the posts were treated in chromic sulfuric acid followed byrinses in distilled water, HCl and isopropyl alcohol. They were thendipped immediately into a solution of the organic molecules. After a fewminutes, the slots were removed and rinsed quickly in alcohol to removemolecules from the surface. Alcohols used in preparation of the slotsinclude isopropyl, ethyl, methyl and butyl alcohol. After rinsing inalcohol, the slots were blown dry with nitrogen to remove excess alcoholand prevent water condensation in the evaporating alcohol layer.Molecules inside the pores were detected using fluorescencespectroscopy. FIG. 7 shows the fluorescence spectra of tetracenemolecules trapped in slots, in a solid and in solution. It can be seenthat the slot spectrum is intermediate between the others; it retainsthe 510 nm peak characteristic of individual molecules yet still hasconsiderable intermolecular interaction as evidenced by the long redtail. Quantitative comparisons of the luminescence intensities showsthat tetracene occupies roughly 50% of the pore volume.

FIG. 8 shows the fluorescence spectrum of dipyrenyl decane moleculestrapped in slots and in solution. The slots were prepared as in Example2 above but they were only 20 Å wide. This data shows that the dipyrenyldecane molecules in the micropores do not combine to form dimers orexcimers (see, e.g., J. B. Birks, Photophysics of Aromatic Molecules,Wiley-Interscience, London, 1970) as they do in solution, as evidencedby the weakness of the 470 nm excimer band. This is due to the confiningeffect of the slot walls.

For such a large fractional filling of the slots, the appearance ofslots in transmission electron microscopy should be quite differentbefore and after filling with molecules. FIGS. 9a and 9b show how theappearance of slots change after absorbing molecules so that a largefraction of the available pore volume is filled. FIG. 9a shows slotsetched into a Si/SiO₂ superlattice using HF and FIG. 9b shows theappearance of the slots after absorbing dipyrenyl decane into slots. Thedifferences seen in the micrographs would not be apparent unless a largefraction of the slot structure were filled with molecules.

To further establish that the molecules observed in the fluorescencespectra were located in the interior of the slot structures, thefollowing experiment was conducted. Tetracene molecules were absorbedinto slot structures and excess material was removed by rinsing withisopropyl alcohol for 20 seconds and blowing off the excess. The samplewas then ion beam etched with oxygen for 10 seconds using a 500 eV, 0.3mA/cm² beam. This selectively removes organic material on the exteriorsurface of the slot structures without substantially affecting the slotsor the material contained therein. The fluorescence spectra was observedto be the same before and after the milling, showing that the moleculesare primarily in the slots.

Many molecules were observed to adsorb into the slots, includinganthracene, tetracene, perylene, chrysene, and Rhodamine B. Rubrene, onthe other hand, was not observed to adsorb into 20 Å slots. This weattribute to the large size (8 benzene rings) and three dimensionalcharacter of the molecule.

The strength of the absorption was observed to vary with the moleculeabsorbed and the micropore material. Some molecules, notably tetracene,were observed to desorb into alcohol solutions only after several hours,a very long time by usual desorption standards. Other molecules desorbedvery quickly, faster than could be measured.

Due to the preparation method, these micropores were presumably linedwith OH functionalities, and are capable ot separations based on thesurface chemistry. Because of the almost unlimited variety of materialsthat can be made into superlattice micropores, the number of surfacetreatments which can be applied is much larger than is possible insilica gels, as an example. By making the slot width comparable tomolecular dimensions of interest, shape and size selective effects canbe introduced into separative processes. Slot width can be madecomparable with molecular dimensions either by directly fabricating thedesired width or by reducing the effective slot width using a coatingformed after the slot is fabricated. Coatings applied to the slot wallwhich reduce the effective width can either be organic or inorganic.Suitable organic molecules for lining the interior of a slot andreducing the effective width include alcohols, surfactants, silanecoupling agents . . . . The organic layer not only reduces the effectivewidth of the two dimensional slot, but also can be used to alter thesurface chemistry. For instance, the wall of a slot can be made eitherhydrophobic or hydrophyllic and exhibit absorption phenomena which arenot only size dependent but are also selective to the hydrophobocity ofa molecule. In existing zeolites the pores are too small to take such alining without clogging.

Larger pores are useful for separations of polymers. Superlatticemicropores are much more monodisperse in pore size than existingsystems. It might also be possible to use the two-dimensinal characterof the slots to make shape selective separations, e.g., to separatebranched from linear polymers.

Since the active material can be fabricated directly on a substrate,several different new types of "fixed" bed separative processes can beenvisioned. As stated above, by making the substrate out of a flexiblematerial such as polyimide, it is possible to make a continuous belthaving microporous superlattice material on the surface. This belt canbe moved between two tanks of liquids, transporting material trapped inthe slots from one tank to another. By absorbing material in one tankand removing it from the slots in another, a continuous separation isproduced.

Another separation method can be designed to take advantage of theplanar geometry, offered by microporous superlattices fabricated onsubstrates. In this case a chromatographic separation column ismicrofabricated using the microporous superlattice as the packing. Bymicrofabricating a flow channel over a microporous superlattice, it ispossible to obtain a micropore chromatography column inexpensively madeusing lithographic fabrication techniques.

What is claimed is:
 1. A process for absorbing a molecular species froma medium of a liquid or a gas comprising contacting said medium with asuperlattice which includes multilayered material having layers whichextend laterally with different dimensions so as to provide twodimensional slots between said layers within said multilayered material.2. The process of claim 1 wherein said molecular species is organic. 3.The process of claim 2 wherein said molecular species is a hydrocarbon.4. The process of claim 3 wherein said molecular species is aromatic. 5.The process of claim 2 wherein said molecular species is selected fromthe group consisting of tetracene, pyrene, anthracene, napthalene,rubene and dipyrenyl decane.
 6. The process of claim 1 wherein saidcontacting step separates a molecular species from said medium.
 7. Theprocess of claim 6 further comprising the step of removing saidsuperlattice from said medium thereby removing said molecular species.8. The process of claim 7 wherein said absorption is shape selective. 9.The process of claim 7 wherein said absorption is size selective. 10.The process of claim 9 wherein a smaller molecule is separated from alarger molecule by the smaller molecule preferentially entering saidslots.